1. Field of the Invention
The present invention relates to methods and devices for manufacturing biaxially oriented tubing of thermoplastic material.
2. Description of the Related Art
When producing biaxially oriented tubing of thermoplastic material, e.g., pipes of polyvinylchloride, it has proven to be difficult to produce tubing with uniform final dimensions. Such uniformity is desirable, e.g., as biaxially oriented tubing elements, e.g., pressure pipes, e.g., for transportation of water, are interconnected end-to-end, e.g., via socket connections.
In EP 823 873 a method is disclosed for the production of biaxially oriented tubing. Use is made of a rigid mandrel having an expansion part as well as a run-on part upstream of and integral with the expansion part. Spaced upstream from the nose end of the run-on part a closure member is held on the anchoring rod, so as to define a chamber in the lumen of the tube in preform condition. A liquid, e.g., heated water, is fed under pressure between the tube and the mandrel device via one or more ducts that are formed in the mandrel and have a port in the outer surface of the mandrel. This liquid then flows counter to the direction of movement of the tube towards the chamber upstream of the nose end of the expansion device and is then discharged via one or more discharge ducts in the anchoring rod.
In EP 823 873 it is also proposed to provide the mandrel with a run-off part downstream of the expansion part. A film of cold liquid is created between the tube and this run-off part, as one or more feed and discharge ducts for the cold liquid are formed in the mandrel. In particular it is proposed to cause the cold liquid in the film to flow opposite to the motion of the tube, so from a downstream feed opening in the outer surface of the run-off part towards an upstream discharge opening in the outer surface of the run-off part.
In EP 823 873 the tube is made to sealingly engage the mandrel at or near the transition between the expansion part and the run-off part in order to avoid that the cold liquid reaches the expansion part.
In EP 1 159 122 a method is disclosed for the production of biaxially oriented tubing. Use is made of a rigid mandrel having an expansion part as well as a run-on part upstream of and integral with the expansion part. The run-on part has a uniform diameter over its length. A film of liquid is formed between the expansion part and the tube. The liquid is supplied at the downstream end of the expansion part and flows counter to the motion of the tube to one or more outlets arranged in the run-on part of the expansion device. The tube in preform condition is shown to sealingly engage on the nose end of the run-on part as the inner diameter of the preform is less than the diameter of the run-on part.
Thus, a need exists for establishing production processes and production installations that allow producing biaxially oriented tubing of thermoplastic material, the oriented tubing having a desired uniformity of the final dimensions of the oriented tubing as well as good strength properties.